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Sky-high tower of power may ride the waves

By Michelle Knott

WHAT’S over 7 kilometres tall and dangles into the North Sea? According to researchers in the Netherlands, it could be MegaPower – an enormous power station which they claim may one day be a major source of pollution-free energy.

While the scheme may sound crazy, the working cycle that the tower would use to generate electricity is similar to the cycle that underlies hydroelectric power. The turbines in a hydroelectric power station harness the potential energy of water as it falls from a lake or reservoir towards the sea. But this is only half the story. The water gained this potential energy when it evaporated from the ocean and rose into the clouds. There it cools down and condenses to form rain that replenishes the lakes.

MegaPower would replace the water with other fluids, and enclose the system inside a giant tower. At the top the fluid would condense in the cold of the upper atmosphere. From there it would fall through a turbine to the bottom of the tower, where heat exchangers would draw in energy from the sea to evaporate the fluid and start the cycle again.

The results of a year-long feasibility study into the idea were presented last month. The study was coordinated by Novem, the Netherlands Agency for Energy and the Environment, on behalf of the Dutch government. Other contributors include the Dutch iron and steel conglomerate Hoogovens, the industrial machinery manufacturer Stork Ketels, and German engineering contractor Linde.

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The study concluded that the thermodynamics of the scheme were sound, and that the vast structure should not be impossible to build. “There’s a lot to find out,” says Ronald Rovers of Novem. “Of course, we haven’t solved every problem yet.”

Novem has not yet calculated the cost of the scheme, but claims it will be “within the costs of providing equivalent conventional generating capacity”. One MegaPower installation would have a capacity of 7000 megawatts. The largest conventional power station in Britain, at Dray in North Yorkshire, is rated at 4000 megawatts.

In its simplest form, the MegaPower design envisages a tower 80 metres in diamenter and 5 kilometers high, that uses a chemical such as ammonia as the working fluid. The tower has several hydrogen tanks to make it buoyant in the air and keep it upright. The base of the structure, suspended in the sea, would be held in place by three 8-kilometre cables. “Iron or steel cables would break under their own weight, but new materials can cope,” says Rovers. To minimise the weight of the tower, it would be built from plastic sandwiched between two skins of aluminium. It would be around 50 metres in diameter and weigh over 400 000 tonnes.

In a second version, the height is increased to 7.5 kilometres, with a second, upper stage that uses hydrogen at a pressure of around 300 atmospheres as the working fluid. At this pressure, the temperature at the top of the tower will be low enough to liquefy the hydrogen. Ammonia vapour arriving at the top of the first stage would give up its heat to the hydrogen, making it evaporate. In this version, the hydrogen in stage two reduces the need for floats. There would still be a need to incorporate hydrogen for buoyancy lower down, but the gas would be held inside the tower itself. At sea level, this version would be twice the diameter of the single-stage version.

The Netherlands National Aerospace laboratory modelled the behaviour of both structures in high winds, using data from the Royal Netherlands Meteorological Institute. It found that they should survive bad weather. “The tower will bend but it will stay in place,” says Rovers.

In spite of encouraging results from the study, MegaPower is unlikely to be built in the foreseeable future. “It’s too big to do with a few people and a few companies,” admits Rovers.

Other engineers are cautious. “It’s an exciting idea, but when you start conceiving of structures that are so huge, you can’t really make reliable comments without doing a detailed study,” says Adrian Fox of the consultant engineering firm Arup. “If feasible, it would require significant breakthroughs in technology.”

He acknowledges that the potential environmental benefits may one day offset the development and capital costs and make the project commercially viable. “Currently this seems unlikely,” says Fox, “given the low cost of conventional energy sources and generating methods.”

Alan Carter, a structural engineer with Amec Process and Energy, was more sceptical&colon; “The scheme for MegaPower would not, in my opinion, be economic until hydrocarbon fuels have been expended.”